Seminar: "Femtosecond laser filamentation in air:
the roles of optical nonlinearity and plasma generation" by Dr. Yu-hsin
Chen
Wednesday, March 12, 2014 11:00 AM to 12:00 PM
CREOL Room 102
CREOL Room 102
Dr. Yu-hsin Chen
Abstract:
An intense femtosecond laser pulse propagating in a gas may
collapse into one or multiple “filaments” when its peak power exceeds the
critical power (5 – 10 GW in air) for nonlinear self-focusing. In atmosphere,
the laser intensity is typically ~ 1013 W/cm2 in the
filament, leaving a weakly-ionized plasma channel which can extend meters in
length with a diameter of < 100 μm. While it has been generally accepted
that laser filamentation is the consequence of self-focusing-induced beam
collapse stabilized by plasma generation and de-focusing, neither the
field-induced nonlinearity nor the plasma generation had been directly
measured. This uncertainty has given rise to recent controversy about whether
plasma generation does indeed counteract the positive nonlinearity, as an alternate
theory suggests that the stabilization mechanism is contributed by saturation
of optical nonlinearity.
For a basic understanding of femtosecond filamentation and
for applications, the focusing and defocusing mechanisms – nonlinear
self-focusing and ionization – must be understood. By employing a single-shot,
time-resolved technique based on spectral interferometry to study the
constituents of air, it is found that the rotational responses in O2
and N2 are the dominant nonlinear effect in filamentary propagation
when the laser pulse duration is longer than ~ 100 fs. Furthermore, we find
that the instantaneous nonlinearity scales linearly up to the ionization
threshold, suggesting that an ionization-free, negative stabilization of
filamentation does not exist. This is confirmed by space-resolved plasma
density measurements in meter-long filaments using optical interferometry with
a grazing-incidence probe laser pulse.
Biography:
Dr. Yu-hsin Chen received the B.S. degree in electrical
engineering and the M.S. in electro-optical engineering in 2000 and 2002,
respectively, both from National Taiwan University. He obtained his Ph.D.
degree in electrical engineering from University of Maryland, College Park in
2011. Then he worked as a postdoctoral researcher at Lawrence Livermore
National Laboratory.
Dr. Chen’s research interests are in ultrafast lasers,
nonlinear optics, high-intensity laser-plasma interactions, and laser
acceleration of charged particles. He has won 2012 Marshall N. Rosenbluth
Outstanding Doctoral Thesis Award in plasma physics, for his work on
femtosecond laser filamentation in atmosphere.
For additional information:
Dr. Martin C. Richardson
Pegasus Professor and University Trustee Chair, Northrop
Grumman Prof of X-ray Photonics; Prof of Optics; Director Townes Laser Institute
407-823-6819
mcr @ creol . ucf . edu
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